• homing pigeon
• hippocampal formation
• memory retention
• GPS data logger
• familiar landmark

The hippocampal formation plays a major role in the memory system, particularly spatial memory, of many taxa. The avian hippocampal formation (HF) is considered the homologous of the mammalian hippocampal formation. Previous studies suggested that lesions to HF do not impair homing pigeons’ ability to determine the home direction when displaced to distant unfamiliar locations where olfactory cues provide navigational information. By contrast, hippocampal lesions seemed to impair localization of the loft in the last phase of the homing process. However, so far a systematic study on memory retention of HF-lesioned pigeons with a controlled pre-operative experience is still lacking. This study aimed at assessing whether and what pigeons subjected to hippocampal lesions can remember about the cues learned during their preoperative homing experience.
The fieldwork was conducted from June to July 2019 using one-year old homing pigeons (n=43) allowed to freely fly around the home area, but never subjected to displacement experiments. The pigeons were released singly and tracked from three different sites: La Costanza (north from home), Livorno (south from home), and Arnaccio (east from home). Following these three baseline releases, pigeons were divided into two groups: control (n=18) and HF-ablated (n=25) pigeons. Control pigeons underwent sham surgery, consisting of deep anesthesia, while HF-ablated pigeons underwent a bilateral aspiration of the hippocampus. After a post-operation resting period, all pigeons were again released singly from each of the above three release sites. For all releases (pre- and post-lesion), each pigeon was tracked with a GPS data logger to record positional data.
For the analysis, each track was divided into three portions: (i) initial decision making phase (DM), from 500 m from the release point up to when the bird reaches 2 Km distance from the release site without flying back towards the release site; (ii) en-route navigation phase (ER), ranging from 2 km from the release site up to 4 km from the loft; (iii) local navigation phase (LN), from 4 Km from the loft up to home. For each portion of pre- and post-lesion track the efficiency index (EI) was computed and compared within and between groups. Furthermore, homeward component, a measure of the orientation of the track towards the goal, was calculated both for the DM+ER section and the LN section of each track, and comparisons within and between groups were performed.
The GPS tracking data showed that after hippocampal lesions the birds were unimpaired in determining the home direction and efficiently approaching the home area. However, HF-lesioned pigeons were dramatically impaired in localizing the loft in the last phase of their homing process, indicating that HF-ablated pigeons were unable to navigate by relying on familiar landmarks near home. In conclusion, the results of the present work add further strength to the notion of the role of the hippocampus in avian navigation in familiar areas.